This invention relates to microelectrodes, in particular diamond microelectrodes.
Microelectrodes, amongst other applications, are used in electrochemical applications to characterise fluids, such as liquids or gases. Such electrochemical measurements can be qualitative or comparative, or they may be quantitative. Quantitative measurements generally require that the system is amenable to mathematical modelling, and in both cases it is desirable that the signal to noise in the system is maximised and that as much information as possible is extracted from the system (see Feeney et al, Electroanalysis, vol 12 (2000), page 677). Both of these objectives can best be achieved by using small electrodes, i.e. microelectrodes, such that the configuration approximates to a semi-spherical or three dimensional diffusion model rather than either a linear or two dimensional diffusional model.
The use of such microelectrodes is well known in the art, and became an active field of research in the late 1970's. Subsequent general development of electronics has provided the tools required to utilise such electrodes efficiently. Typical benefits realised include increased temporal resolution, increased current density, decreased sensitivity to solution resistance, and steady state diffusion profiles.
Traditionally, such microelectrodes are arranged as an array presenting contact surfaces that are exposed to the fluid to be analysed. Typically, in order to fabricate such microelectrodes a conductive electrode material, such as a metal, is coated with a non-conducting layer, which is then perforated with one or more apertures to form the microelectrodes which will come into contact with the fluid. In recent times, boron doped CVD diamond has become established as an electrode material, and fabrication of microelectrodes onto a boron doped diamond layer has been reported. Typically such electrodes are a few microns in diameter, fabricated by applying a layer of Si3N4 or similar non-conductive material to the surface of the diamond and subsequently etching apertures into it to expose the diamond underneath (e.g. P Rychen et al., Electrochemical Society Proceedings, Vol. 23 (2001), pp 97-107). More recently, the present applicant developed a microelectrode comprising a diamond layer formed from electrically non-conducting diamond and containing one or more pins or projections of electrically conducting diamond extending at least partially through the layer of non-conducting diamond and presenting areas of electrically conducting diamond, as disclosed in international patent application WO 2005/012894. The use of an electrode formed solely of diamond provides: (i) exceptionally high resistance to attack by a very wide range of chemicals under a very wide range of conditions, (ii) a wide potential window and (iii) low background currents allowing the devices to be used and remain stable in a wide range of chemically aggressive environments.
However, a two-dimensional diamond microelectrode array (“MEA”) such as that described in WO 2005/012894 has a number of drawbacks. For instance, the method of fabrication of the devices is complex and requires precision bulk removal of the boron-doped material to leave the array of pins, a subsequent second deposition step to refill the space between the pins with intrinsic (i.e. non-conductive) diamond, and a final step in which the surface is precision polished to re-expose the tops of the pins. There is also a necessity for the spacing between individual electrodes to be greater than the diffusion length of the species being investigated so that each device can be treated as being independent of all the surrounding devices. The overall signal level may also be low as the density of active sites may be low. Generally the devices also need to operate under conditions where diffusion is the dominant mechanism for species transport (i.e. they need to operate under quiescent conditions).